Weighing apparatuses, in particular, electronic weighing apparatuses are used in a variety of different fields. One important field of application is the so-called pipette calibration. During the pipette calibration process a sample vessel is positioned on the sample holder of a weighing apparatus and filled step by step via a pipette, wherein the fill quantity of each filling step is measured gravimetrically and is compared with the corresponding nominal values of the pipette, so that the accuracy of the pipette can be checked or, more specifically, adjusted. In order to fill the sample vessel, it is absolutely necessary that the pipette enters into the weighing chamber at least in certain regions. On the other hand, it is necessary to protect the weighing chamber from external interferences. In particular, air flows and temperature variations can distort the weighing result. These problems that are known from conventional electronic balances are usually solved with a closed, so-called draft shield, a weighing chamber floor and a weighing chamber lid, i.e., with a protective housing that completely surrounds the weighing chamber. In order to position and manipulate the sample to be weighed, it is also known to design at least one wall of the protective housing in such a way that it can be opened and closed, as required. The problem of accessibility of the weighing chamber becomes more critical for pipette calibration stations, i.e., for weighing apparatuses specialized for the pipette calibration process, due to the fact that the conventional procedures for calibrating a pipette require that a very large number of filling operations of the sample vessel be carried out in succession in a very short period of time. To open an entire wall, for example, to open a section of the draft shield for this purpose, would be associated with excessive disturbances of the atmosphere in the weighing chamber.
In EP 1 715 312 B1 it is proposed to provide a small opening in a wall of the draft shield that is just large enough to guide a pipette or any other manipulation tool to the sample vessel on the sample holder. This opening in the known apparatus can be closed or is to be opened with a movable closure element, which may be in the form of a slider, a flap or an iris. The published document also mentions in passing the possibility of a motorized drive of the closure element, so that the provision of a control unit for controlling the motor is also implicitly disclosed. However, the inconvenient requirement that the operator has to initiate a command to open or close the closure element for each pipetting operation is a problem.
It is known from DE 203 16 286 U1 to provide for this purpose a non-contact switch in the form of a light barrier. However, it has the disadvantage that the operator is forced to always execute the same sequence of motions during the pipetting operation, i.e., in particular, the passing through the light barrier. For extensive pipetting operations this may be ergonomically stressful.
DE 203 04 465 U1 discloses an analytical balance that has a draft shield with motor-driven wall elements, wherein the motor is controllable by way of an antenna for querying a transponder. In this case it is necessary that the respective transponders be attached to the samples to be weighed or to the sample containers.
Therefore, the aforementioned generic document provides to use a reflection sensor, for example, an infrared or ultrasonic sensor, with a sensitivity range having defined limits and to couple this reflection sensor to the control unit in such a way that the entry of an object, such as, for example, the tip of a pipette, into the sensitivity range causes an opening and a removal of the object from the sensitivity range and a closing of the closure element. Since the reflective properties of different objects vary per se and also as a function of their orientation and motion, the sensitivity range of the sensor that is relevant for the closing or opening is not clearly defined. This can lead to a faulty actuation of the closure element. There is a distinct learning curve before the operator is able to develop a feel for the motions that can be performed without thereby activating the sensor or the motions that have to be performed so that the sensor is activated. However, the possible or rather necessary motions and the body postures are not perceived by all operators to be ergonomically comfortable over long periods of time. In addition, the extent of the sensitivity range may also change due to aging or fouling of the sensor components.